MXene Key Composites: A New Arena for Gas Sensors

Highlights
 With its layered structure, abundant functional groups, and excellent electrical conductivity, MXene is of great research interest in the field of gas sensing. The preparation technology of gas sensors is constantly being optimized, opening up avenues for the development of gas sensing. MXene-based composite materials (MXene/graphene, MXene/metal oxides, MXene/MOF, and MXene/polymer) are applied in various gas sensors.

ABSTRACT With the development of science and technology, the scale of industrial production continues to grow, and the types and quantities of gas raw materials used in industrial production and produced during the production process are also constantly increasing.These gases include flammable and explosive gases, and even contain toxic gases.Therefore, it is very important and necessary for gas sensors to detect and monitor these gases quickly and accurately.In recent years, a new two-dimensional material called MXene has attracted widespread attention in various applications.Their abundant surface functional groups and sites, excellent current conductivity, tunable surface chemistry, and outstanding stability make them promising for gas sensor applications.Since the birth of MXene materials, researchers have utilized the efficient and convenient solution etching preparation, high flexibility, and easily functionalize MXene with other materials to prepare composites for gas sensing.This has opened a new chapter in high-performance gas sensing materials and provided a new approach for advanced sensor research.However, previous reviews on MXene-based composite materials in gas sensing only focused on the performance of gas sensing, without systematically explaining the gas sensing mechanisms generated by different gases, as well as summarizing and predicting the advantages and disadvantages of MXene-based composite materials.This article reviews the latest progress in the application of MXene-based composite materials in gas sensing.Firstly, a brief summary was given of the commonly used methods for preparing gas sensing device structures, followed by an introduction to the key attributes of MXene related to gas sensing performance.This article focuses on the performance of MXene-based composite materials used for gas sensing, such as MXene/graphene, MXene/Metal oxide, MXene/Transition metal

Introduction
As a key component in information acquisition and signal conversion, sensors play an irreplaceable role in Internet of Things technology [1][2][3][4].Among them, gas sensing can convert gas molecular signals over optical signals, electrical signals, etc., widely applicable to aerospace, industrial production, agricultural planting, and human health monitoring, to realize monitoring, forecasting, and automatic control of toxic and harmful gases, as well as prediction of human respiratory system diseases [5].Sensors are mainly classified into electrical (resistive/capacitive), electrochemical, mass-sensitive, and optical types [6].Electrical gas sensors are widely studied due to their simple structure and easy processing of output signals.Gas sensing materials in electrical sensor components can adsorb gas molecules through physical/chemical interactions and undergo charge transfer, thereby causing changes in the electrical signal of the device.Currently, electrical gas-sensitive materials include metal oxide semiconductors (MOSs), precious metals, carbon materials, organic materials, and two-dimensional materials.Since the discovery of graphene, two-dimensional materials, such as transition metal chalcogenides (TMDs) [7], boron nitride (BN) [8], layered double hydroxides (LDHs) [9], black phosphorus (BP) [10], and transition metal carbon/ nitrides (MXenes) [11], have also been applied in the field of gas sensing.MXenes have become an emerging gas sensing material due to their unique layered structure, significant physical, optical, and electrical properties, as well as active surfaces [12,13].
In recent years, the rapid development of MXenes has led to their rapid application in the field of gas sensing (Fig. 1) [1,35,38].MXenes-based gas sensors are expected to achieve efficient and rapid detection of gases such as ammonia (NH 3 ), nitrogen dioxide (NO 2 ), and volatile organic compounds (VOCs) at room temperature [42][43][44].However, due to its excellent electron transfer performance, two-dimensional layered structure, and abundant terminal groups, MXenes are not only sensitive to inorganic gases prone to electron loss or capture, but also highly sensitive to volatile organic compounds such as alcohols, ketones, and aldehydes [45].This results in poor selectivity and specificity of MXenes in gas detection.Therefore, researchers often use surface modification, doping, and composite methods to enhance the gas sensing characteristics of MXenes [46][47][48][49][50][51][52].Among them, compounding is an important strategy [53,54].The gas-sensitive composite phases of MXenes mainly include graphene and its derivatives, metal oxides, TMDs, MOFs, and polymers [55][56][57].
The current paper reviews the recent research progress of MXenes-based composites for gas sensors.Figure 2 shows an overview of the review article, highlighting the preparation of gas sensors, with a focus on the synthesis, advanced performance, and gas sensing behavior of MXenes composite materials (MXene/graphene, MXene/ metal oxides, MXene/transition metal sulfides (TMDs), MXene/metal-organic framework (MOF), MXene/polymer).Finally, the (potential) advantages and challenges related to the development of MXenes were systematically discussed.
Fig. 1 A significant research schedule of MXene compounded with other materials for gas sensors includes MXene self-modification, MXene/ graphene, MXene/metal oxide, MXene/transition metal sulfide, MXene/MOF, MXene/polymer, etc. Reproduced with permission from Refs.[1,35,38,[42][43][44] received attention and favor from researchers [67,68].Gas sensors can detect various gases, such as gas composition detection in chemical production, coal mine gas concentration detection and alarm, environmental pollution monitoring, gas leakage, fire alarm, combustion detection, etc.With the continuous development of social technology, the types of substrates continue to increase.However, traditional methods for preparing gas sensing devices are not suitable for many substrates, and traditional methods require high preparation conditions, low production efficiency, and extremely high preparation costs [69][70][71][72][73][74].Therefore, innovative manufacturing technologies for gas sensors are very important [75].Appropriate manufacturing methods for gas sensors have provided strong support for the wide application of gas sensors by not only improving the performance of the sensors, effectively simplifying the process steps and reducing the cost of production [76][77][78].
Coating technology is a simple and efficient way to prepare sensitive soluble materials into thin-film structures at the surface of a substrate [115,116].The preparation process of this technique is not demanding in terms of equipment and fabrication conditions, making it suitable for many substrates, including many flexible substrates, especially if the boundary range of the sensitive material film is not strictly required [76,117].Therefore, the coating technique is also one of the most prevalent methods for the fabrication of gas sensors today.Specific methods of coating technology include trickle coating, spin coating, spray coating, and dip coating.Trickle-coating method is one of the simplest ways to prepare gas sensors by selecting a soluble sensitive material and applying the material solution dropwise onto the substrate via a pipette, which is simple to operate; spincoating is an alternative and convenient method of making sensitive films, in which a soluble material is dripped onto a rotating substrate, which is then dried so that the substrate vaporizes the solvent and a sample of the substrate containing a thin film of the sensitive material is obtained; spraycoating method can be assisted by ultrasound and combines the hydrophilic and hydrophobic properties of the materials adhered to prepare a homogeneous and sensitive film, and it is a cost-effective method [79,80,118].However, its shortcoming is that for films with specific needs (e.g., specific requirements for shape and location), a concealment procedure had to be applied to the areas that did the coating not need to be applied; dip coating is a versatile and costeffective method of preparing gas-sensitive sensors, which proceeds by dipping the substrate into a solution of sensitive material, then adjusting the speed to lift the substrate out of suspension, and finally drying to eliminate any residual solvent on the substrate surface.
Printing technology is an innovative and modern manufacturing technology that enables the preparation of functional material suspensions based on substrates using the appropriate printers and finds its application in a wide range of electronic manufacturing applications [81,119,120].Predesigned gas sensors, such as specific patterns, film thicknesses, and boundary ranges, can be prepared on a massive scale by printing technology.Printing technologies can be categorized into four main types, which are inkjet printing, screen printing, writing printing, and nano-imprinting.Inkjet printing is an intriguing digital method for contactless spraying of ink and functional materials onto a variety of substrates through micron-sized nozzles; screen printing is recognized as a highly attractive and competitive manufacturing technology compared to inkjet printing for the rapid mass fabrication of microelectronic devices due to its predesigned grid pattern and ease of manufacturing process; writing printing on a substrate is a familiar and practical printing technique in which a combination of a functional material solution is deposited on the substrate to form a structure by combining it with a pen or any other writing instrument; nanoimprint lithography (NIL) is a lithographic technique that offers the advantages of high productivity, low cost, and simplicity of the process to fabricate nanostructures in high volume, high resolution (< 5 nm), and lower cost.Simply put, NIL technology uses high-resolution electron beams and other methods to pattern complex nanostructures on a stamp, and then deforms the sensitive material with the patterned stamp to form the patterned material.Unlike traditional photolithography (where the direction or energy of the ions of the sensitive material is altered by photons or electrons to achieve pattern production), NIL technology mechanically deforms the sensitive material through direct contact, thus avoiding the resolution limitations of traditional techniques such as light diffraction or beam scattering.
Spinning is the process of extracting a precursor functional solution (e.g., polymer solution or melt) from a nozzle and depositing it on a collector to create long, continuous, one-dimensional fibers with micron/nanometer diameters [121][122][123].Textiles can be coated with sensitive materials to form gas sensors.In addition, electronic devices based on sensitive optical fibers can be directly fabricated for gas detection via incorporating gas-sensitive materials into the precursor solution, which is a straightforward and effective method.Of the various spinning technologies, the electrospinning technology is of great interest and is widely utilized for the preparation of a wearable device.
Many substrates are incompatible given that in particular conventional fabrication techniques (e.g., chemical vapor deposition (CVD)), certain substrates cannot withstand drastic fabrication conditions (e.g., high temperatures, chemical etching reagents).The optimum way to resolve these incompatibilities lies in the transfer of nanostructures or thin films on rigid/donor substrates (e.g., silicon, glass) prepared by conventional fabrication techniques to acceptor substrates (e.g., PET, PMDS), which is defined as a transfer technique [124,125].Effective transfer techniques are critical to the fabrication of flexible gas sensors, which will enable many traditional fabrication processes that are only applicable to hard substrates to be used in the manufacture of wearable/ flexible sensors [114].Transfer techniques consist of dry transfers, wet transfers, and support coatings-assisted transfers.Dry transfer utilizes the adhesion gap between the film layer and the underlying substrate to transfer the film from the primary substrate to the intended substrate; wet transfer is available for transferring a mono sensitive layer to a variety of substrates in service media; and support layer-assisted transfer is a prominent transfer technique that utilizes an elastomeric impression as a support layer to retrieve a material with micro/nanostructures back from the supplier substrate and attach it to a non-natural substrate [126].However, most of the aforementioned widely practiced techniques (e.g., coating, printing, and spinning) rely on the sensing material being in the liquid phase, this restricts the amount of gas sensing materials available because some types of materials with excellent sensing capabilities are harder to realize in the bulk of the liquid phase.Spin-coating and screen-printing methods result in ink waste due to the use of solution-phase materials, while inkjet printing and electrospinning processes both require the use of nozzle devices, with the risk of nozzle clogging.Moreover, technology of transfer, particularly transfer with the assistance of a supporting layer where at least two etching cycles are involved, is partly complex and time-consuming.Hence, a long way lies ahead in commercializing the product for the exploitation of gas sensors with enhanced performance and large-scale production.

Structure of MXene
MXene material is a type of metal carbide or metal nitride material with a two-dimensional layered structure.It is a two-dimensional transition metal group carbon/nitride obtained by selectively etching the A atomic layer in the ternary conductive ceramic MAX phase.The phase structure of MXene is shown in Fig. 4a, and the general formula of MXene structure is M n+1 X n T x , where M is a transition metal (such as Ti, V, and Mo), X represents C or N, n = 1, 2, or 3, T x represents surface terminal groups (-OH, = O, and/or -F) [127].Due to the hexagonal crystal structure formed by the interlacing of the M layer and X layer with the A layer in the precursor MAX phase of MXenes, the MXene phase also has a similarly symmetrical hexagonal lattice (Fig. 4b).The M atoms in MXenes are arranged in a tight structure, while the X atoms fill the gap positions of the octahedron.There are three arrangements in the MXenes structure: [128].As shown in Fig. 4c, the two-dimensional MXene consists of a thin sheet that has hexagonal cells, with an X layer sandwiched between two M transmission metal layers.

MXene Characteristics for Gas Sensing
In the aqueous environment of chemical etching solutions, the outer surface of the detached MX layer is usually functionalized by -F, -OH or = O functional groups.These surface rich functional groups (-F, -OH or = O) can become attachment sites for the direct growth of other nanostructured materials or functional molecules [129,130], which can be modified to provide feasibility for improving the selectivity of gas sensors.In addition, this surface functionalization has a significant impact on the electronic and ion transport properties of MXenes, namely, the conductivity of MXenes is directly related to the electron transfer process that occurs on their surface [131].MXenes have certain metal properties and narrow bandgap semiconductor properties, which give them the inherent advantage of good conductivity.For example, Ti 3 C 2 T x has a room temperature conductivity of up to 10,000 S cm −1 [132,133].
Many theoretical calculations have shown that the ideal MXene is located near the Fermi level, with a considerable electron density and a cash property [128,130].Lane et al. calculated the ideal single-layer defect free MXene nanosheets using density functional theory, and the results showed that MXene exhibits metal conductivity, with Fermi levels higher than its precursor MAX phase [131].However, when its surface is functionalized, some MXenes exhibit semiconductor properties.In addition, due to the different number of electrons received by different surface groups (-F, -OH, or = O) in equilibrium states, different surface groups have different effects on the electronic properties of MXene, and the orientation of the end groups also affects the electronic properties of MXene [132].Table 2 lists the bandgap widths of some MXenes.MXenes with different bandgap widths can be used to prepare gas sensing arrays, achieving specific recognition of industrial raw gas, exhaust gas, and human exhaled gas.In summary, using MXenes as a gas sensing material has certain inherent advantages.

MXenes Composite in Gas Sensing Applications
In recent years, MXene composite materials containing graphene, semiconductor metal oxides, transition metal sulfides, organic metal frameworks, polymers, and other materials have received increasing research in gas sensing  1 3 applications.Due to the more metallized nature and narrow band gap of MXenes, the addition of metal oxides, graphene derivatives, and chalcogenides provides more activated adsorption sites, defects, and modulation of working functions, thereby improving gas sensing performance.Table 3 summarizes the performance of gas/VOC/humidity sensors for MXene-based composites.

MXene/Graphene
Graphene is widely used in various fields because of its excellent thermal conductivity, high specific surface area, and easily modified structure [139][140][141][142][143]. MXenes are an excellent sensing material with a very narrow bandgap, but when pure MXenes are used in gas sensing devices, critical potential barriers are generated during the gas reaction process, which hinders their further sensitive response.Subsequently, researchers found that combining the two can effectively overcome this problem.For example, Liu et al. prepared three-dimensional (3D) hybrid aerogel [140] (Fig. 5a) from MXene (Ti 3 C 2 T x ), reduced graphite oxide (rGO) nanosheets, and ultrafine CuO nanoparticles.From the obtained 3D MXene/rGO/CuO aerogel, high pyruvic sensing performance was demonstrated at ambient temperature (Fig. 5b).Response of the sensor to 100 ppm of acetone was 52.09% (RT) (Fig. 5b), with a response time of ~ 6.5 s and a recovery time of ~ 7.5 s (Fig. 5c), demonstrating excellent reproducibility and selectivity.In 2020, Lee et al. [139] developed a Ti 3 C 2 T x MXene/graphene hybrid fiber wearable gas sensor without a metal binder through a wet spinning process (Fig. 5d).The bandwidth capacity of the composite material has increased from 1.05 to 1.57 eV, while the fiber properties of the composite material enhance flexibility and response to NH 3 .A moderate response (6.8% at 50 ppm NH 3 ) was displayed by the composites (Fig. 5e), with this being 7.9 and 4.7 times more responsive than that of pure MXenes and rGO, respectively (Fig. 5f).Wang et al. [141] proposed an ionic conductive composite film, which is composed of reduced graphite oxide (rGO), nitrogen doped MXene Ti 3 C 2 T x (N-MXene), and titanium oxide (TiO 2 ) (Fig. 5g), and detects 4-40 ppm formaldehyde HCHO vapor at room temperature (20 °C) and humidity.In various humidity conditions toward 4 ppm HCHO, the ternary sensor achieved an average reversible response of 26% at 54% RH (Fig. 5h).In addition, it also shows good repeatability, long-term stability, and selectivity (Fig. 5i).The excellent gas sensing performance of rGO nanosheets can be attributed to three aspects: firstly, in humid environments, rGO nanosheets serve as a good conductive platform for transporting and collecting charge carriers; second, the layered N-MXene facilitates the co-sorption and spreading of HCHO and water moieties; third, the TiO 2 nanoparticles provide abundant resorption sites, which promote decomposition of the sorbed water.

MXene/Metal Oxide
Metal oxides represent the oldest and most widely used sensing material and can be used in a variety of applications due to the high specific surface area, ease of fabrication, ease of functionalization, and extremely high sensitivity to a broad range of gases/volatile organic compounds.The sensing mechanism of metal oxides is mainly due to the changes in resistance caused by pre adsorbed oxygen species (oxygen molecules (O 2 ), lattice oxygen < including surface lattice oxygen and bulk lattice oxygen > (O 2− ), atomic adsorption of oxygen (O − ), molecular adsorption of oxygen (O 2 − )), and surface reactions of gas molecules [185].Due to the high dependence of oxygen ionization on operating temperature, this mechanism typically requires metal oxide gas sensors to operate at relatively high temperatures, which is also the main drawback of metal oxide gas sensors [53, [145][146][147]186].However, research data suggests that the mixture of metal oxides with 2D MXenes has a more robust gas/volatile organic compound sensing response, and the emergence 1 3 of this complex greatly overcomes the low selectivity and high operating temperature limitations of pure metal oxide sensing (Fig. 6).Titanium dioxide (TiO 2 ) is an ideal material for gas sensor preparation due to its pollution-free properties, ability to generate photogenerated electrons when stimulated, and simple preparation process.However, TiO 2 -based gas sensors also have some drawbacks, such as poor sensing performance, long response time, and recovery time.In 2019, Tai [144] designed a gas sensing element based on a TiO 2 /Ti 3 C 2 T x bilayer film (Fig. 7a).According to the results, when compared with the pure Ti 3 C 2 T x sensor, this TiO 2 /Ti 3 C 2 T X sensor exhibited a larger recognition value (1.63 times) with shorter response/recovery time (0.65/0.52 times) compared to the pure Ti 3 C 2 T X sensor for 10 ppm NH 3 at room temperature of 25 °C (60.8% relative humidity) (Fig. 7b, c).Choiet et al. [44] covered the amplification and inductive properties against NO 2 by Ti 3 C 2 through the modulation of the introduction of the Schottky barrier (SB) (Fig. 7d), which combines TiO 2 into conducting MXenes to form a heterogeneous structure.The TiO 2 /Ti 3 C 2 composite sensor shows a NO 2 sensitivity 13.7 times higher than the original Ti 3 C 2 MXene (Fig. 7e), while the response of the reducing gas is almost unchanged, the reason for this is the highest charge density of NO 2 in other interfering VOCs due to the formation and movement of SB inside caused by the adsorption of NO 2 molecules, together with other interfering VOCs, and as explained in the mechanisms of sensing (Fig. 7f, g).Kuang et al. [154] successfully prepared Ti 3 C 2 T x -TiO 2 nanocomposites with regular morphology using Ti 3 C 2 T x as the titanium source through a simple one-step hydrothermal synthesis method (Fig. 7h).Due to the formation of interface heterojunctions and modulation of carrier density, the detection response of Ti 3 C 2 T x -TiO 2 sensors to various VOCs at room temperature is enhanced by about 1.5-12.6times compared to pure MXene sensors.In addition, this nanocomposite sensor has a better response to hexanal (the Ti 3 C 2 T x -TiO 2 sensor has a gas response of approximately 3.4% to 10 ppm hexanal).
In addition to the hydrothermal partial oxidation of Ti 3 C 2 T x mentioned above, researchers have also prepared partially oxidized Ti 3 C 2 T x by heat treatment at 350 °C [148] (Fig. 8a-g) and microwave-activated oxygen plasma [149] treatment (Fig. 8h).Sun et al. [150] investigated the processing-dependent sensing behavior of Ti 2 CT x (LiF/HCl), Ti 2 CT x (HF), and TiO 2 /Ti 2 CT x (LiF/HCl) at room temperature under 365 nm ultraviolet light (Fig. 9a, b).In addition, the results indicate that TiO 2 /Ti 2 CT x (LiF/HCl) exhibits better sensing performance than other samples (Fig. 9c).Since it contains abundant oxygen functional groups (-O x , -(OH) x and Ti-O-Ti), providing more NH 3 molecular interactions.Li et al. [159] developed a humidity sensor by in situ growth of TiO 2 nanowires on two-dimensional (2D) Ti 3 C 2 MXene using alkaline oxidation method (Fig. 9d).They found that the sea urchin-like Ti 3 C 2 /TiO 2 composites have an order of magnitude larger surface area when compared to pure Ti 3 C 2 or TiO 2 materials (Fig. 9e) and exhibit documented high sensitivity at environments with low thermal relative humidity (RH) (from 7% RH to 33% RH, approximately 280 pF/% RH) (Fig. 9f).
CuO exhibits the advantage of wide range response to VOCs, but has the drawbacks of small response values, slow response/recovery speed, and low durability.For this reason, Angga Hermawan et al. [145] reported a simple method to prepare CuO-Ti 3 C 2 T x MXene hybrid by self-assembling 1 3 electrostatically (Fig. 10a).CuO-Ti 3 C 2 T x MXene showed a better methane gas sensing response (R g /R a ) of 11.4 than pristine CuO nanoparticles at 250°C for 50 ppm toluene gas sensing response nearly five times higher than that of pristine CuO particles for 50 ppm toluene at 250 °C.(Fig. 10b).In addition, due to the high conductivity of the metal phase in Ti 3 C 2 T x MXene, the hybridization of CuO with Ti 3 C 2 T x MXene not only improves the response time, but also improves selectivity, response (270 s), and recovery time (10 s) (Fig. 10c, d).
Sun et al.In 2019, Sun et al. [187] used a simple solvothermal method to grow one-dimensional W 18 O 49 nanorods in situ on the Ti 3 C 2 T x surface.The W 18 O 49 /Ti 3 C 2 T x composites exhibited a high response to low concentrations of acetone (11.6-20 ppm) (Fig. 11a), as well as high selectivity, long-term stability, and also a fast response and recovery response to very low of acetone (170 ppb) can also be detected (Fig. 11b).Physical properties of NH 3 sensing were improved by forming heterojunctions and enhancing the number of active sites, relative surface area, and pore size of pristine Ti 3 C 2 T x by functionalizing Ti 3 C 2 T x with WO 3 nanoparticles by a simple ultrasonic method, as shown in Fig. 11c.The resulting Ti 3 C 2 T x /WO 3 -50% (weight percent of WO 3 ) sensor exhibited excellent response to NH 3 (22.3% at 1 ppm), which was 15.4 times that of the pristine Ti 3 C 2 T x Fig. 6 MXene/metal oxide for gas sensors.Reproduced with permission from Refs.[44,146,155,158,187] (2024) 16: 209 209 Page 14 of 42 https://doi.org/10.1007/s40820-024-01430-4© The authors sensor (1.54% at 1 ppm) with no electrical resistance drift (Fig. 11d) [151].
In addition to this, He et al. [152] successfully synthesized two-dimensional (2D) MXene modified by tin dioxide nanoparticles for gas sensing detection as well by hydrothermal method.Wang et al. [153] successfully synthesized SnO-SnO 2 (p-n junction) and Ti 3 C 2 T x MXene nanocomposites for gas sensing by a one-step hydrothermal method.Zinc oxide (ZnO) has long been used as a gas detector.Although it has good response to various gases, high operating temperature limits the widespread application as a gas sensing material.Qui et al. [157] ZnO/Ti 3 C 2 T x MXene nanocomposite composed of 2D multilayer MXene and 1D ZnO nanoparticles prepared a room temperature toxic gas sensor (Fig. 12a).The nanocomposite material exhibits enhanced response and recovery behavior to toxic gases, superior to pure Ti 3 C 2 T x MXene and pure ZnO.Its gas sensing principle is shown in Fig. 12b.Under the irradiation of the sun, ZnSO 3 nanocube and layered Ti 3 C 2 T x MXene were synthesized by simple static self-assembly to synthesize ZnSO 3 /Ti 3 C 2 T x MXene nanocomposites (Fig. 12c).Sima et al. [158] found that the ZnSO 3 /Ti 3 C 2 T x MXene nanocomposite-based sensor displayed significant selectivity for formaldehyde, with high response (194.7% to 100 ppm and 62.4% to 5 ppm) (Fig. 12d), and rapid response/recovery times (6.2/5.1 s at 100 ppm formaldehyde) (Fig. 12e), and these tests were Reproduced with permission from Ref. [148].h Schematic diagram of the possible gas sensing mechanism of Ti 3 C 2 T x MXene.Reproduced with permission from Ref [149] conducted at RT. Figure 12f shows the gas sensing scheme of the ZnSO 3 /Ti 3 C 2 T x MXene laminate.
Different research groups have conducted extensive research on gas sensing performance using pure Ti 3 C 2 T x and its complexes with different metal oxides, such as V 2 O 5 [45], -Fe 2 O 3 [155], In 2 O 3 [156], and K 2 Ti 4 O 9 [160].
Liu [155] successfully prepared heterogeneous composite materials of -Fe 2 O 3 and Ti 3 C 2 Tx MXene using a simple hydrothermal method (Fig, 13a-c), and characterized their morphology and microstructure through various characterization methods (Fig. 13b-f).The results indicate that a size of approximately 250 nm wide was prepared -Fe 2 O 3 nanocube and uniformly distributed on the surface of Ti 3 C 2 T x MXene nanosheets.The results indicate that a size of approximately 250 nm wide was prepared -Fe 2 O 3 nanocube and uniformly distributed on the surface of Ti 3 C 2 T x MXene nanosheets.The gas sensitivity test results show that compared with other typical gases, the sensor based on -Fe 2 O 3 /Ti 3 C 2 T x MXene composite material exhibits excellent selectivity toward acetone, and very favorable response to 5 ppm acetone: 16.6% (Fig. 13g), high rate of response and recovery: 5/5 s (Fig. 13h), excellent linearity, and significant repeatability at room temperature (RT) (Fig. 13i) [160].
Taken together, as shown in Table 3, TiO 2 is the most commonly used metal oxide composition to assist MXenes in detecting reducing gases at room temperature.For the response of NH 3 gas, MXenes, and tungsten tin oxides showed the best response values, but the lower response limit did not change significantly, and the response recovery time needs further investigation [164,192].On the other hand, we found that Co 3 O 4 and ZnO are suitable support materials for the detection of oxidizing gases using MXenes.In

MXene/TMDs
Two-dimensional chalcogenides are two-dimensional materials with unique structures, excellent mechanical, electrical, optical properties, and low energy consumption.It is a well-explored sensing application material.However, for gas/ VOC sensing, the research on the composite materials of 2D chalcogenides and MXene is still a rarely explored field.So far, there are only two reports on the combination of MXene and sulfides for gas sensing.Firstly, Qui et al. [161] prepared MoS 2 /Ti 3 C 2 T x heterostructures with interconnected network nanostructures through a simple hydrothermal method (Fig. 14a).The synthesized MoS 2 /Ti 3 C 2 T x heterostructure exhibits significant lattice matching (Fig. 14b), where vertically arranged MoS 2 nanosheets grow on Ti 3 C 2 T x MXene and have a large specific surface area.The obtained gas sensor exhibits very high sensitivity and selectivity to NO 2 gas exposure, reaching up to 25% at 10 ppm, as well as rapid recovery and long-term stability (Fig. 14c, d).Due to the large number of Mo active sites and the conductivity of Ti 3 C 2 T x MXene, which can accelerate electron movement and excellent heterojunction interface contact, the presented structure exhibits enhanced NO 2 sensing activity.Secondly, Chen et al. [162] reported on the Ti 3 C 2 T X /WSe 2 nanohybrid material, which was prepared through simple surface treatment and peel-based  [158] process (Fig. 14e), and combined as a sensing material into inkjet printing and wireless operation sensors (Fig. 14f).The sensing measurement has excellent repeatability and reproducibility.The energy band diagram of the Ti 3 C 2 T x /WSe 2 sensor in the presence of ethanol shows n-type sensing behavior and Schottky barrier modulation (Fig. 14g).Compared with sensors made from raw Ti 3 C 2 T x and raw WSe 2 , the Ti 3 C 2 T x / WSe 2 hybrid sensor exhibits a 12-fold improvement in ethanol sensitivity, low electrical noise, sound selectivity, and ultra-fast response/recovery characteristics (Fig. 14h).Table 3 summarizes a detailed overview of sensors for MXene and TMDs composite materials.

MXene/MOF
In recent decades, metal organic frameworks (MOFs) have developed rapidly and their popularity has not decreased, making them a hot topic in the field of materials.However, the conductive MOF obtained by combining MOF and MXene breaks the shackles of MOF materials that are almost non-conductive, perfectly combines the controllable structure of organic materials and the long-term order of inorganic materials, plus the unique high electron mobility, conductive MOF can be described as a favorite, and is also one of the most potential materials in gas sensing applications [163], such as Chang et al. [164] designing and preparing a rod-shaped porphyrin based metal oxide (Co TCP (Fe)) and MXene (Ti 3 C 2 T x ) through hydrogen bonding to 1 3 form a chemically resistant NO sensing hybrid (Co-TCPP (Fe)/Ti 3 C 2 T x ) (Fig. 15a).The sensor based on Co TCP (Fe)/ Ti 3 C 2 T x shows excellent NO sensing performance at room temperature (Fig. 15b), including high response (= 2.0, 10 ppm) (Fig. 15c), reliable repeatability, high selectivity, low actual detection limit (pLOD, 200 ppb), and rapid room temperature NO sensing response/recovery speed (95/15 s, 10 ppm) (Fig. 15d).

MXene/Polymer
Polymers have excellent flexibility, favorable sensitivity, appropriate electrical conductivity, low cost, a large number of organic groups to interact with the gas on the surface, light weight, and low reaction temperature, making them suitable for gaseous/VOC sensing applications when mixed with MXenes.MXene/polymer composite sensors are used to identify ammonia [101,103,165,166], ethanol [32,105], methanol [33,104], acetone [31,102], and humidity [99,100,112,113] for wear and tear [189][190][191][192][193][194].With respect to ammonia identification, the original MXene-based sensor shows excellent NH 3 sensing characteristics, but ammonia has very high adsorption energy and NH 3 is difficult to partition from the MXene screen during recovery, and demonstrates extended recognition time as well as wandering of the baseline resistance.To surmount these limitations, Li et al. [165] developed in situ a flexible chemorepulsive gas sensor based on a hybrid polyaniline (PANI)/Ti 3 C 2 T x sensitive layer for tracking ammonia volatilization out of agriculture using self-assembled method in situ (Fig. 16a).The sensor exhibits excellent NH 3 sensing performance over a temperature range of 10-40°C at 20%-80% relative humidity (RH) (sensing response to 10 ppm ammonia peaks at 4.7 at 40% RH, which is almost three times higher than in dry air (~ 1.6)) (Fig. 16b-d).Zhao et al. [105] also used over-PANI, via a low-temperature in situ polymerization method to rationally modified PANI particles coated with Ti 3 C 2 T x nanosheets (Fig. 17a,  b).This evoked remarkable detection sensitivity, a rapid response/recovery rate and mechanistic stability as well at room temperature.A year later, Zhao et al. [166] also developed room temperature nanocomposites based on 2D MXenes materials and cationic polyacrylamide (CPAM) (Fig. 17c) with high gas responsiveness and flexibility aimed at building high-performance ammonia sensors.
Conductive polymers-3,4-ethylenedioxythiophene (EDOT) and poly(4-styrenesulfonate) (PSS) are also commonly used to composite with MXene.Jin et al. [103] used a dip coating technique to make a gas sensor from the resulting PEDOT:PSS/MXene composite (Fig. 18a).NH 3 at room temperature demonstrated a strong gas response of 36.6% to 100 ppm NH 3 with recovery and response times of 116 and 40 s.Furthermore, the hybrid sensor presented stronger sensitivity performance compared to pure PEDOT:PSS and Ti 3 C 2 T x MXene-based sensors, evidencing that the PEDOT:PSS copolymer and Ti 3 C 2 T x MXene two-dimensional ingredients have a synergistic effect on each other.In addition to showing a high response to ammonia gas, it also responded well to other gases, e.g., Wang et al. [104] used a 4:1 mixture of PEDOT:PSS and Ti 3 C 2 T x to prepare a methanol gas sensor (Fig. 18b, c), where the reaction rate of 5.54 was high for the largest reaction and the second largest reaction tested at room temperature when compared to pure PEDOT:PSS and pure Ti 3 C 2 T x .
For humidity sensing, composites of polymers with MXene are excellent materials.The synergistic effect of chitosan-modified Ti 3 C 2 T x exhibited remarkable performance, enhancing the electrical response to H 2 O molecules.Inspired by the structure of onions (Fig. 19a), Li and colleagues [112] synthesized ion-excited MXene/chitosan-quercetin multilayer membranes (MCQMs) using a layering-by-layer assembly approach (Fig, 19b, c) for which strong interactions to the molecules were found (Fig. 19d).The monolayer pair exhibited the highest resistance in MCQMs, with improved conductivity and reproducibility as the number of layers increased, and the sensor exhibited an ultra-high responsiveness (317% at 90% RH), a wide field of detection, and praiseworthy response and recovery speeds (0.75 and 1.6 s at 90% RH) (Fig. 19e-g).For true breathing studies, An et al. [110] described the mechanism of aqueous adsorption of a multilayer component made of MXene microsheets with polyelectrolytes (Fig. 19h) intended for super-fast humidity sensing (Fig. 19i-k), and they showed that MXene/polyelectrolyte multilayers prepared using layerby-layer (LbL) components exhibited response and recovery times exceeding those of most humidity sensors (Fig. 19l,  m).

Other Materials
Li et al. [111] fabricated a transparent mobile hygrometer using an inkjet printing technique, using a Ti 3 C 2 /Ag blend as a humidity-sensitive membrane and polydiallyldimethylammonium chloride-based (PDDA) as an adhesive barrier (Fig. 20a).The sensor has ultra-high sensitivity (106 ± 800%) (Fig. 20d), fast responsiveness (80 ms), and excellent resistance to bending (Fig. 20c, d).Liu et al. [105] reported a vacuum-assisted layer-by-layer assembly  20e) for conformal deposition of conductive materials on textiles (Fig. 20f, g), resulting in a leaf like nanostructure composed of silver nanowires (AgNWs) as high conductivity skeletons (veins) and transition metal carbide/carbon nitride (MXene) nanosheets as thin layers.Having a highly sensitive humidity response (57% RH) (Fig. 20h), Zhu et al. [97] demonstrated a new paper thin-film H 2 sensor using Ti 3 C 2 T x MXene nanosheets and palladium colloidal nanoclusters (Pb CNC) as activators.The MXene@Pd CNC paper film is easily prepared through a vacuum filtration process based on a fully colloidal solution (Fig. 21a).The paper film is flexible, lightweight, and has a dense, shiny surface.The obtained MXene@Pd CNC thin-film sensor exhibits moderate H 2 response at room temperature in a flat or curved state (Fig. 21b).Specifically, MXene@Pd CNC thin-film sensor provides a response time of (32 ± 7)s and a sensitivity of S = (23.0± 4.0)% ± 4% H 2 (Fig. 21c).In addition, the MXene@Pd CNC sensor can perform "in situ mode" H 2 detection directly along a paper film of the required size.Intense H 2 entrapment in the ultrafine palladium carbon nanotube lattice alters the work function and leads to MXene's electron codoping, explaining the underlying regime of gas induction (Fig. 21d).Muckleyet et al. [110] reported on ion intercalated MXenes (Ti 3 C 2 -K and Ti 3 C 2 -Mg) for humidity sensing (RT).Ion embedding increases the spacing between MXene layers and absorbs H 2 O molecules between the layers (Fig. 21e).The conclusion drawn from neutron scattering combined with theoretical calculations is that K + and Mg 2+ ions cause each ion to embed 2 and 5 H 2 O molecules, respectively, indicating  1 3 an increase in lattice parameters.They also found that the weight response of MXene to water is 10 times faster than their electrical response, indicating that the expansion/contraction of channels between MXene layers caused by H 2 O leads to the capture of H 2 O molecules as depletion charge dopants (Fig. 21f-i).
Within other studies, the investigators tried to improve the sampling performance by doping iron molybdate (Fe 2 (MoO 4 ) 3 ) [107], Ni(OH) 2 [106] and Ti 3 C 2 T x MXene for H 2 (in room temperature), n-butanol (in 120 ℃), and NH 3 (in room temperature) sensing, respectively.In another study, transition metal fluoride oxide (TiOF 2 ) was surface modified on Ti 3 C 2 T x and subsequently used as a humidity sensor (Fig. 22a-f).By stabilizing the surface end groups, the MXene films showed improved reaction area, flexibility, and catalytic oxidation (Fig. 22j).In addition, the manufactured sensors exhibit good sensitivity and selectivity when exposed to humid environments [98] (Fig. 22h, i).Table 3 provides a detailed overview of sensors based on MXene nanocomposites.In conclusion, insertion of metallic ions and precious metals is also an effective way to improve the gas sensing performance of the original MXenes [109].[112].h Schematic of the PDAC/MXene assembly.i Schematic illustrations showing the proposed humidity response mechanism of the MXene/polyelectrolyte multilayers.Schematic diagrams of MXene/polyelectrolyte multilayers and the corresponding electrical circuit models for j low and k high humidity.l and m Comparison of recovery and response times between the MXene/polyelectrolyte multilayers from this study and other humidity sensors reported in the literature.Reproduced with permission from Ref. [110] https://doi.org/10.1007/s40820-024-01430-4© The authors The new MXene gas sensor will be the next generation of universal sensors for future wearable electronic devices, with performance comparable to other 2D material sensors.Through Table 3, it can be clearly found that most of the reported 2D MXenes-based composites are suitable for sensing at room temperature.Secondly, MXenes-based composite materials have been tested for sensing different gases/VOCs and have been found to be highly sensitive to ammonia, acetone, ethanol, nitrogen dioxide, methane, and humidity.On the other hand, the application of MXenebased composites in gas sensors has advantages and disadvantages, as shown in Table 4. ).Reproduced with permission from Ref. [111].e Schematic illustrating the fabrication of hydrophobic, permeable, and conductive silk textile with a vacuum-assisted layer-by-layer assembly approach.f Schematic of the MAF silk detecting sweating humidity.g Humidity response of (MA) n F silk for monitoring human sweating.h Sensitivities of electrical resistance change at 57% RH for MAF silk.(MAF: MXene/Ag NWs/POTs).Reproduced with permission from Ref. [108] 1 3

MXenes Surface Adsorption Calculation
It has been theoretically proven that MXenes with semiconductor properties (M 2 CO 2 , M = Sc, Ti, Zr, Hf) are highly sensitive to NH 3 , as shown in Fig. 23a.Xiao et al. [216] calculated and found that after NH 3 was adsorbed as an electron donor on M 2 CO 2 , charge transfer mainly occurred between the M atom of M 2 CO 2 and the N atom of NH 3 .When MXene adsorbed NH 3 , the charge of NH 3 molecules was transferred to the transition metal atom on the surface of MXene, and the conductivity of Ti 2 CO 2 was significantly improved.They also found that desorption of NH 3 can be easily achieved by adjusting the electrons injected into M 2 CO 2 , making the NH 3 sensor reversible [217].For example, the lowest unoccupied electronic state (LUES) of Zr 2 CO 2 mainly comes from Zr atoms, which means that when an additional electron is introduced into Zr 2 CO 2 , the electrons will fill the unoccupied electronic orbitals of Zr atoms.Therefore, the injected electrons are mainly distributed on the transition metal, leading to an increase in the metal bond length and adsorption  as shown in Fig. 23b, c.The surface functional groups of MXenes have an undeniable contribution or impact on gas sensing performance.Junkaew et al. [219] used density functional theory (DFT) calculations to investigate the reactivity and selectivity of four O-functionalized MXenes, namely M 2 CO 2 (M = Ti, V, Nb, Mo), toward gas molecules.According to the calculated adsorption energy results, among the 11 gas molecules, Ti 2 CO 2 and Nb 2 CO 2 have stronger adsorption capacity for NH 3 , while Mo 2 CO 2 and V 2 CO 2 are more sensitive to NO.The surface functional groups of Ti 3 C 2 T x MXene material are a combination of -F, = O, and -OH.The presence and content changes of these functional groups can achieve selective sensing of gas molecules.For example, Pourfath et al. [220] studied through charge difference calculations that the contribution of surface functional groups to charge transfer is different.Fluorine atoms have a smaller contribution to charge transfer than oxygen atoms.Therefore, there is a strong electrostatic attraction between the lone pair electrons of the O atom in the = O functional group on MXenes and the positively charged part of the exposed hydrogen atom in NH 3 molecules.Therefore, controlling the content of the = O functional group on the MXenes surface can improve the selectivity toward NH 3 molecules.Recently, Naqvi et al. [221] explored several gases (such as CH 4 ) through DFT calculations.

First Principles Exploration of MXenes Gas-Sensitive Mechanisms
Maleski et al. [42]   The interlayer spacing of Ti 3 C 2 T x membrane decreased by 0.51 Å compared to that after ethanol swelling, as shown in Fig. 23e.Therefore, regulating the interlayer distance of Ti 3 C 2 T x MXene is also very important for improving the selectivity of gas sensing.

Summary and Outlook
Starting from the application of new MXene-based composites in the field of gas sensing, this article briefly introduces the preparation methods of gas sensing devices, the structure of MXene, and the properties related to gas sensing.It focuses on the research progress of MXene and graphene, metal oxides, TMDs, MOFs, and polymers in the field of gas sensing, and summarizes the gas sensing mechanism of MXene.However, the development of practical gas sensors based on MXene still faces many challenges: 1.It is necessary to develop green and safe macro preparation methods and surface functional group oriented regulation technologies for MXene.At present, the most mature preparation method for M n+1 X n is liquidphase chemical etching, usually using ternary M n+1 AX n precursors as starting materials.In fluorinated solutions such as hydrofluoric acid (HF) and fluoride salts (LiF + HCl, NH 4 HF 2 ), chemical etching selectively removes the A-layer elements in ternary M n+1 AX n , achieving good selective etching effect and obtaining functional group-rich multilayer M n+1 X n T x materials.On the one hand, MAX phase is usually formed through high-temperature processing of titanium and aluminum, and requires several grinding processes to obtain fine MAX powder.On the other hand, using hydrofluoric acid or fluorinated salts as etching solvents, the highly toxic gases generated during the preparation process seriously endanger human and environmental safety.In addition, the etching capabilities of different solution systems vary, resulting in low two-dimensional yield and difficulty in optimizing the preparation process.This will result in high preparation costs for MXene materials and limit their large-scale application in the gas sensing field.More importantly, the fluorine containing solution reaction system inevitably leads to the random coexistence of three functional groups (= O, -F, -OH) on the surface of M n+1 X n T x , making accurate control extremely difficult.The regulation of functional group states (types and quantities) by changing experimental conditions faces enormous challenges in experiments, and mature and feasible experimental methods for precise regulation of functional groups have not yet been formed, making it difficult to improve selectivity for specific gases through the design of surface functional groups.2. The variety of MXene material systems still needs to be greatly expanded.Since the discovery of MXene materials in 2011, people's understanding of their structure is still in the initial stage, especially the lack of effective preparation techniques for the types of MXene materials predicted by theory.As a result, MXene currently used in the gas sensing field mainly focuses on twodimensional Ti 3 C 2 T x and its composite materials.For the large number of MXene material families, more innovative preparation methods have been developed to synthesize pure MXene materials with more diverse types.The combination of surface modification, element doping, heterogeneous recombination and other means to design the composition of the material is a technical bottleneck in expanding the application of MXene in the gas sensing field.3. The interaction mechanism between MXene and gas molecules needs to be further studied.Due to the richer atomic species and combination types of MXene compared to traditional two-dimensional materials such as graphene, the surface adsorption and charge transfer mechanisms in gas-sensitive processes will be more complex.Whether it is oxidizing or reducing, it is observed that all adsorbed gas molecules will cause an increase or decrease in resistance with a high signal-tonoise ratio.At the same time, interlayer expansion also has a significant impact on the conductivity changes and gas response of the material.
At present, research on MXene is still in its infancy, providing a basic building block for gas-sensitive materials.Experimental data and computational predictions indicate that by selecting over 60 sets of available layered ternary carbides and nitrides, stable structures of different types of MXene can be obtained.It is expected that MXene and its composites will have unlimited potential in the field of gas sensing. (

Fig. 4 a
Fig.4a Explain the "M," "A," and "X" elements of MAX phase through the periodic table, as well as the schematic diagram of MXenes structure and the currently reported MXenes.Reproduced with permission from Ref.[127].b Crystal structure of MXene generated from MAX phase.c Side views of pristine M 3 X 2 , M 4 X 3 , M' 2 M"X 2 , and M' 2 M" 2 X MXenes, where M, M', and M" denote transition metals, and X represents C or N. Reproduced with permission from Ref.[128]

Fig. 5 a
Fig. 5 a Schematic illustration of fabrication process of 3D MXene/rGO/CuO aerogel.b The selectivity for 3D MXene/rGO/CuO aerogel-based sensor to different gases of 100 ppm at RT. c Resistance changes of 3D MXene/rGO/CuO aerogel when exposed to 100 ppm acetone at RT.Reproduced with permission from Ref.[140].d Schematic illustration of the spinning process for MXene/GO hybrid fiber.e Comparison of the gas response of MXene film, rGO fiber, and MXene/rGO hybrid fiber (40 wt% MXene).f Gas selectivity comparison of rGO fiber and MXene/ rGO hybrid fiber (40 wt% MXene) to various testing gases at concentrations of 50 ppm.Reproduced with permission from Ref.[139].g Schematic images of IDEs sensor.h Sensing performance of the ternary sensor toward HCHO vapor under 54%RH at 20 °C.i Selectivity investigation among a series of interference gases under 54%RH at 20 °C.Reproduced with permission from Ref.[141] [146] used simple noncovalent chemical methods and hydrothermal methods to effectively rivet Co 3 O 4 nanocrystals onto functionalized Ti 3 C 2 T x MXene sheets of branched polyethylene imine (PEI), and prepared Co 3 O 4 @ PEI/Ti 3 C 2 T x MXene composite material (Fig. 10e).Sun et al. examined the sensing performance of nitrogen oxides (consisting of NO 2 and NO) using Co 3 O 4 @PEI/Ti 3 C 2 T x (CoPM) complexes and found that CoPM-24 complexes exhibited 27.9% response when added at 2.4 mg Ti 3 C 2 T x along with high selectivity and very weak detection limits (30 ppb-NO x ) (Fig. 10f, g).In 2021, Zhang et al. [147] as a high-performance self-powered formaldehyde (HCHO) sensor based on MXene/Co 3 O 4 composite was prepared.Electricity was supplied through piezoelectric nanogenerators (PENGs) of ZnO/MXene nanowire arrays.p-type metal oxide Co 3 O 4 provided more active sites for formaldehyde interactions, thus the MXene/Co 3 O 4 composite exhibited good response at room temperature with 9.2% response at 10 ppm HCOH and low detection limit-0.01ppm.

Fig. 7 a
Fig. 7 a Structure of TiO 2 /Ti 3 C 2 T x Gas Sensor.b Normalized response recovery curves of the TiO 2 /Ti 3 C 2 T x , Ti 3 C 2 T x and TiO 2 gas sensors to 10 ppm NH 3 .c Response/recovery times of the Ti 3 C 2 T x and TiO 2 /Ti 3 C 2 T x gas sensors.Reproduced with permission from Ref. [144].d A diagram of the composition process of TiO 2 /Ti 3 C 2 MXene sensor.e Experimental real-time gas response curve of TiO 2 /Ti 3 C 2 depending on NO 2 concentration.f and g Suspension regime of NO 2 gas by Ti 3 C 2 and TiO 2 /TiO 2 /Ti 3 C 2 thin films.Reproduced with permission from Ref.[44].h Schematic diagram illustrating the process of the Ti 3 C 2 T x -TiO 2 nanocomposites preparation and gas sensing device fabrication.Reproduced with permission from Ref.[154]

Fig. 8
Fig. 8 Characterization of gas sensing in partially aluminized Ti 3 C 2 T x MXene films on a multisensor chip.a Stabilization of the aqueous solution of layered MXene in photographic form.b Diagram of the construction of a single-layer Ti 3 C 2 T x MXene sheet.c Photographs of an AFM image of a Ti 3 C 2 T x MXene flake on Si/SiO 2 , the scale bar is 1 μm m. d Multi-electrode chip scheme for MXene sheet membrane produced by drop-in casting method.e SEM image of the MXene films overlaying the area in contact between the platinum alloy electrode and the Si/SiO 2 backing.Scale bar is 1 μm.f MXene partial conductivity G(t) variation, relative to conductivity in air (in dry air at 350 °C with acetone, isopropyl alcohol (IPA), ethanol and methanol dosed sequentially at 2-10 ppm).g Illustration: dependence of chemical reactions, S = ΔG/Gair, average of all MXene sensor elements for organic vapor focus on a chip; error bands indicate the fluctuation of resistance of the entire multisensor array.Reproduced with permission from Ref.[148].h Schematic diagram of the possible gas sensing mechanism of Ti 3 C 2 T x MXene.Reproduced with permission from Ref[149]

Fig. 10 a
Fig. 10 a Schematic Representation of a Facile Preparation of CuO nanoparticles/Ti 3 C 2 T x Hybrid Heterostructures and Gas Sensor Device Fabrication.b Gas sensing response of CuO, Ti 3 C 2 T x MXene, and CuO/Ti 3 C 2 T x MXene tested at different working temperatures.c Response/ recovery times.d Selectivity of CuO/Ti 3 C 2 T x -30 wt% to 50 ppm of tested gas.Reproduced with permission from Ref. [145].e Schematic illustration of the Co 3 O 4 @PEI/Ti 3 C 2 T x MXene composites.f CoPM-24 sensor selectivity study under the influence of the presence of various gases at 100 ppm.g Momentary feedback of CoPM-24 (Co 3 O 4 @PEI/Ti 3 C 2 T x ) sensor to 100-0.03ppm NO x .Reproduced with permission from Ref.[146]

Fig. 11 a
Fig. 11 a Image demonstrating the mechanism of acetone sensing by W 18 O 49 /Ti 3 C 2 T x nanocomposites.b Image of instantaneous response curves of W 18 O 49 /Ti 3 C 2 Tx -based sensors in the range of acetone concentrations from 0.17 to 500 ppm.Reproduced with permission from Ref.[187].c Mechanisms of sensing of NH 3 by Ti 3 C 2 T x /WO 3 -50% of composites.d Different amounts of WO 3 of the composite sensor in response behavior to 1 ppm NH 3 at room temperature.Reproduced with permission from Ref.[151]

Fig. 12 a
Fig. 12 a Schematic synthesis procedure of ZnO/Ti 3 C 2 T x heterostructure.b Schematic NO 2 -sensing reaction mechanism of ZnO/Ti 3 C 2 T x nanocomposite.Reproduced with permission from Ref. [157].c Schematic of fabrication process of (c 1 ) ZnSnO 3 nanocube, (c 2 ) layered Ti 3 C 2 T x MXene and (c 3 ) ZnSnO 3 /Ti 3 C 2 T x MXene composites.d Selective curve of ZnSnO 3 /Ti 3 C 2 T x MXene composites to 100 ppm different gas at room temperature.e Response performance of ZnSnO 3 /Ti 3 C 2 T x MXene composites to 100 ppm form aldehyde at room temperature.f Schematic of gas sensing mechanism of ZnSnO 3 /Ti 3 C 2 T x MXene composites.Reproduced with permission from Ref.[158]

Fig. 13
Fig. 13 Illustration of the preparation process of a positively charged α-Fe 2 O 3 nanocubes, b Sheet-like Ti 3 C 2 T x MXene and c α-Fe 2 O 3 /Ti 3 C 2 T x MXene composites.d SEM images of α-Fe 2 O 3 /Ti 3 C 2 T x MXene composites.e TEM image.f HRTEM image of the α-Fe 2 O 3 /Ti 3 C 2 T x MXene composites.Reproduced with permission from Ref. [155].g Selective property of the sensor based on α-Fe 2 O 3 /Ti 3 C 2 T x MXene composites to 5 ppm of various target gases at room temperature.h The real-time resistance measurement of α-Fe 2 O 3 /Ti 3 C 2 T x MXene composite sensor toward acetone vapor at RT. i Long-term stability of the α-Fe 2 O 3 /Ti 3 C 2 T x MXene-based sensor for 5 ppm acetone.Reproduced with permission fromRef.[160]

Fig. 14 a
Fig. 14 a Schematic illustrating the synthesis process of the MoS 2 /Ti 3 C 2 T x heterostructure from the Ti 3 AlC 2 MAX phase.b HRTEM images of the MoS 2 /Ti 3 C 2 T x heterostructure.c Comparison of responses of MT2 sample to various gases at 10 ppm concentration.d Cyclic responses of MT2 to 10 and 20 ppm NO 2 gas.Reproduced with permission from Ref. [161].e Schematic illustration of preparation processes for Ti 3 C 2 T x / WSe 2 nanohybrids.f Schematic illustration of inkjet-printed gas sensors in detection of volatile organic compounds with a wireless monitoring system.g Comparison of gas response as a function of ethanol gas concentrations for Ti 3 C 2 T x and Ti 3 C 2 T x /WSe 2 sensors.h Selectivity test of the Ti 3 C 2 T x and Ti 3 C 2 T x /WSe 2 sensors upon exposure to various VOCs at 40 ppm.Reproduced with permission from Ref.[162]

Fig. 15 a
Fig. 15 a Synthesis process of Co-TCPP(Fe), Ti 3 C 2 T x , and Co-TCPP(Fe)/Ti 3 C 2 T x .b Schematic diagram of the sensing mechanism of the Co-TCPP(Fe)/Ti 3 C 2 T x -20 toward NO.c Selectivity of the sensor to various gases at concentrations of 10 and 20 ppm.d Real-time response-recovery curve of the Co-TCPP(Fe)/Ti 3 C 2 T x -20 based sensor toward 10 ppm NO at room temperature.Reproduced with permission from Ref.[164]

Fig. 16 a
Fig. 16 a Application scenarios of PANI/Ti 3 C 2 T x hybrid sensitive film-based flexible NH 3 sensor for ammonia volatilization monitoring in agriculture.b Selectivity of the hybrid sensor to NH 3 and other interference gases in agricultural fields at room temperature.c Moisture dynamic response of the NH 3 sensing performance of PANI/Ti 3 C 2 T x hybrid sensitive films.d Dynamic sensing response of the hybrid sensor toward 10 ppm NH 3 in the range of 10-40 °C at dry air and 60% RH.Reproduced with permission from Ref.[165]

Fig. 17 a
Fig. 17 a A diagram of the composite synthesis of PANI/Ti 3 C 2 T x nanocomposite, which includes the peeling process of Ti 3 AlC 2 and the consolidation process of ANI.b Sketch of the Inter-digital polarization of the electrodes shown before and after plating PANI/Ti 3 C 2 T x nanocomposites.Reproduced with permission from Ref.[105].c Synthesis scheme of CPAM/Ti 3 C 2 T x nanocomposites, including the etching process for Ti 3 AlC 2 and composite process of CPAM and Ti 3 C 2 T x .Reproduced with permission from Ref.[166]

Fig. 18 a
Fig. 18 a Schematic Illustration for the Synthesis of PEDOT:PSS/MXene Composites and the Fabrication Process of the Composite-Based Gas Sensor.Reproduced with permission from Ref. [103].b Ti 3 C 2 T x /PEDOT:PSS profile of material and gas sensor manufacturing.c The diagram of the experimental setup.Reproduced with permission from Ref. [104]

Fig. 19 a
Fig. 19 a Photograph of purple onion scale leaves and schematic diagram of the scale leaves.b Schematic diagram of the MCQMs composed of MXene flakes and chitosan-quercetin membranes.c The humidity sensor based on laser-induced interdigitated electrode upon PI substrate.Inset shows the photograph of the flexible humidity sensor.d Chitosan and H 2 O intercalation induced by MCQMs.e 4-Layer induction response to MCQMs.f and g A study of the reaction/recovery time of four layers to MCQM under diverse humidity conditions.Reproduced with permission from Ref.[112].h Schematic of the PDAC/MXene assembly.i Schematic illustrations showing the proposed humidity response mechanism of the MXene/polyelectrolyte multilayers.Schematic diagrams of MXene/polyelectrolyte multilayers and the corresponding electrical circuit models for j low and k high humidity.l and m Comparison of recovery and response times between the MXene/polyelectrolyte multilayers from this study and other humidity sensors reported in the literature.Reproduced with permission from Ref.[110]

Fig. 20 a
Fig. 20 a Flowsheet for the fabrication of Ti 3 C 2 /Ag-based moisture sensor by inkjet publishing method.b Characteristics of the TA2 response and recovery of the sensor exposed to varying relative humidity (RH) conditions.c Duration of response and recovery of sensor TA2.d Application of sensor TA2 to various curvature measurement performance.(TA2: Ti 3 C 2 /Ag = 2wt%).Reproduced with permission from Ref.[111].e Schematic illustrating the fabrication of hydrophobic, permeable, and conductive silk textile with a vacuum-assisted layer-by-layer assembly approach.f Schematic of the MAF silk detecting sweating humidity.g Humidity response of (MA) n F silk for monitoring human sweating.h Sensitivities of electrical resistance change at 57% RH for MAF silk.(MAF: MXene/Ag NWs/POTs).Reproduced with permission from Ref.[108]

Fig. 21 a
Fig. 21 a Diagrams of the manufacturing of MXene and MXene@Pd CNC films and photographs of the completed Pd CNC and MXene suspending solutions.b Sensitivities and response times of MXene@Pd CNC film sensor to 4% H 2 (left) and the corresponding flexibility show (right) under different bending angles.Sensitivities and response times of MXene@Pd CNC film sensor to 4% H 2 after n-time bending cycles (left) and one bending cycle show from =0° to 180° and back to 0°. c MXene@Pd CNC film real-time response/recovery profiles for a wide range of high H 2 compositions (0.5 ~ 40 v/v%).d Band diagrams of Pd and MXene before and after being exposed to H 2 , and electronic transfer between the surface H 2 sorbed and Pd CNC and MXene.Reproduced with permission from Ref.[97].e Design the structure of MXenes interaction between water vapor and ion insertion.f The normalized elastic strength of mature MXene samples measured at 2 K increments over a temperature interval of 20 to 300 K. g A representative normalized QENS spectrum was measured at 300 K from the same sample with a representative Q = 0.51 Å −1 .h Dependence of half-width at half-maximum extracted from the model fit on Q 2 Solids lines are jump diffusion model fits.The extracted water diffusion coefficient values are shown.i Elastic time constants for the reactions of Δ R and Δ M during H 2 O desorption ( ).Reproduced with permission from Ref.[110]

Fig. 22 a
Fig. 22 a Scheme for the fabrication of TiOF 2 @Ti 3 C 2 T x .b and c The cross section of the monolayer of the TiOF 2 @Ti 3 C 2 T x sheet and the rainbow map to show the composition distribution in situ.d TEM image of TiOF 2 nanospheres growing on the Ti 3 C 2 T x substrate.e HRTEM image of TiOF 2 nanospheres.f HRTEM image of Ti 3 C 2 substrate.g Scheme for the hydrolysis and adsorption to synthesize TiOF 2 @Ti 3 C 2 T x .hComplex impedance property of TiOF 2 @Ti 3 C 2 T x at the different RH.i Response and recovery properties of sensors with TiOF 2 , Ti 3 C 2 T x and TiOF 2 @Ti 3 C 2 T x .j Three samples tested for extended stability at variable humidity.Reproduced with permission from Ref.[98]

Fig. 23 a
Fig. 23 a Side and top views of the most stable configurations of different gas molecules adsorbed on the Ti 3 C 2 O 2 surface.Reproduced with permission from Ref. [216].b Two-probe model of monolayer Sc 2 CO 2 sensor for detecting SO 2 molecule.c Predicted I-V characteristics of Sc 2 CO 2 with SO 2 molecules.Reproduced with permission from Ref. [218].d Density functional theory (DFT) simulation results for gas molecules adsorbed on various 2D materials.Side and top views of the minimum energy configurations for acetone and ammonia on Ti 3 C 2 (OH) 2 .Minimum binding energies of acetone and ammonia on Ti 3 C 2 (OH) 2 , Ti 3 C 2 O 2 , Ti 3 C 2 F 2 , graphene, MoS 2 , and BP.Reproduced with permission from Ref. [42].e The (002) peak shift of Ti 3 C 2 T 2 film during N 2 purging for 200 min.The (002) peak shift of Ti 3 C 2 T x film during introduction of CO 2 (1%) or ethanol (0.1%) for 70 min, followed by N 2 purging for 120 min to purge out target gases.Reproduced with permission from Ref. [222].

Table 1
Summarized the technology for preparing gas sensing equipment, summarized its advantages and disadvantages, as well as the demand for materials

Table 2
Bandgap width of some MXenes

Table 3
Gas/VOC/humidity sensor based on MXene-composite

Table 3
(continued) [222]DFT to simulate and calculate the binding energies of acetone and ammonia on Ti 3 C 2 T x , MoS 2 , RGO, and BP to study the sensing mechanism of Ti 3 C 2 T x on acetone and NH 3 gases, as shown in Fig.23d.For the two gases of acetone and ammonia, Ti 3 C 2 (OH) 2 exhibits the strongest binding energy more than twice that of other two-dimensional materials.It is speculated that the superior gas adsorption performance of hydroxyl groups in Ti 3 C 2 T x is the main reason for its high sensitivity to acetone and ammonia.This work demonstrates the presence of charge transfer induced by gas adsorption in the gas sensing mechanism of MXenes.In addition, Zhou et al.[216]used Ti 3 C 2 T x as a gas sensing material to test CH 4 , H 2 S, H 2 O, NH 3 , NO, ethanol, methanol, and acetone gases at room temperature, and found that Ti 3 C 2 T x had very high selectivity for NH 3 .In order to understand the reason for this high selectivity, they also studied the adsorption behavior, adsorption energy, adsorption geometry, charge transfer, and other aspects using first principles calculation methods.They also confirmed that the charge transfer caused by NH 3 adsorption on Ti 3 C 2 T x is the main reason for the change in resistance of Ti 3 C 2 T x .However, MXenes have metal conductivity and contain interlayer water molecules, which means that gas molecules may interact in a more complex manner than typical charge transfer.Koh et al.[222]demonstrated the swelling effect of gas on Ti 3 C 2 T x MXene materials by intercalating Ti 3 C 2 T x with Na + ions and using in situ XRD technology.After 70 min of ethanol blowing, the (002) peak of Ti 3 C 2 T x shifted toward a smaller angle and the interlayer spacing increased by 0.82 Å.After 120 min of N 2 blowing, the adsorbed ethanol was desorbed and the (002) peak of Ti 3 C 2 T x recovered toward a larger angle.